Thermoplastic vulcanizates (“TPVs”) comprise blends of dynamically cured elastomers and thermoplastic resins, where the elastomer may be dispersed within the thermoplastic resin phase as finely-divided elastomer particles. TPVs have advantageously demonstrated many of the properties of thermoset elastomers, yet are processable as thermoplastics. As such, elastomeric articles are often extruded from TPVs using thermoplastic extruding techniques.
Metal oxides, such as zinc oxide, are often used in the production of TPVs. The metal oxide can act as a reaction moderator and/or as a heat aging stabilizer for the TPV. For example, PCT Publication No. WO2005/028556 and U.S. Pat. No. 7,439,304 B2 describe compositions comprising (i) a dynamically-cured rubber, (ii) from about 20 to about 300 parts by weight of a thermoplastic resin per 100 parts by weight rubber, (iii) from 0.2 to 0.9 parts by weight stannous chloride per 100 parts by weight rubber, and (iv) from 0.25 to 4.0 parts by weight metal oxide per 100 parts by weight rubber.
However, in the process for making the TPV the metal oxide, such as zinc oxide, is usually fed to the reactor/mixer in the form of powder. It has been observed that metal oxide powder, such as zinc oxide powder, can have an affinity for the metal surface of the feeder and a strong tendency for compaction within the feeder. This can result in inconsistent addition of the metal oxide to the extruder and in some cases a fouled or plugged feeder. In such cases, removal of blockages within the feeder may result in stoppage of the TPV production line, or may require a new feeder for replacement. Additionally, certain metal oxide powders that have a smaller particle sizes may be even more difficult to handle.
Inconsistent dispersion of the metal oxide into the TPV production process, may also affect the cross-linking of the elastomeric phase during vulcanization. As such, the resulting TPVs may have reduced cross-link density and increased oil swell.
Therefore, there is a need for methods of making TPVs that minimize or eliminate issues arising from addition of metal oxide powders, and to increase the consistency of the addition of the metal oxide to the process. There is also a need to provide methods of making TPVs that produce TPVs having improved cross link density and/or exhibit decreased oil swell.